1. Field of the Invention
The present invention relates to a method and a device for measuring the flow rate of fluid. The method and device of the present invention are suitably used for measuring the flow rate of liquified petroleum gas (LPPG) used for an automobile.
2. Description of the Prior Art
In the prior art device for measuring the flow rate of a gas, a heater coil and temperature sensing coils are arranged outside the flow rate measuring pipe through which the gas to be measured is passing. The heater coil is located between the upstream temperature sensing coil and the downstream temperature sensing coil. The heat produced from the heater coil and supplied to the flowing gas creates a temperature difference between the upstream gas and the downstream gas. The upstream gas temperature is detected by the upstream sensing coil, and the downstream gas temperature is detected by the downstream sensing coil. The mass flow rate of the gas through the flow rate measuring pipe is measured from the difference between the temperatures detected by the upstream and downstream sensing coils.
A problem with the prior art device is that the speed of response to changes in the flow rate of the gas is relatively slow, e.g., 3 seconds. Another problem is that the inner diameter of the flow rate measuring pipe through which the gas to be measured passes is relatively small, e.g., 0.7 mm, hence the passage of the gas through the flow rate measuring pipe is susceptible to dust, contamination, or the like. The problem is particularly serious in the case of LPG, which contains tar impurities apt to be deposited on the inner wall of the flow rate measuring pipe, often blocking the passage of the LPG.
A further problem is that the specific heat of LPG changes in accordance with the change of temperature, yet no correction is carried out with respect to the change of the specific heat of LPG due to the temperature.